Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In practical use, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
In the fuel cell, sealless structure is often adopted. In the sealless structure, a fuel gas such as a hydrogen gas is supplied to the anode, and an oxygen-containing gas such as the air is supplied to the cathode. The remaining fuel gas after consumption in the power generation reaction (off gas) and the oxygen-containing gas are discharged to the outside from the outer circumferential portion of the fuel cell. At this time, in the air discharged to the outside of the fuel cell, back diffusion to the anode may occur. Consequently, the backwardly diffused air and the fuel gas supplied to the anode may cause combustion reaction undesirably.
In this regard, for example, a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2005-85521 is known. As shown in FIG. 25, the fuel cell comprises a power generation cell 1 including a solid electrolyte layer 1a, and a fuel electrode layer 1b and an oxidizing gas electrode layer 1c on both surfaces of the solid electrolyte layer 1a. Further, a fuel electrode current collector 2 and an oxidizing gas electrode current collector 3 are provided for the power generation cell 1, and separators 4 are provided outside the fuel electrode current collector 2 and the oxidizing gas electrode current collector 3 to form the fuel cell having sealless structure. An insulating cover 5 having a gas discharge hole 5a is provided to cover the outer circumferential portion of the fuel electrode layer 1b and the fuel electrode current collector 2.
According to the disclosure, since the insulating cover 5 covers the outer circumferential surface of the fuel electrode current collector 2, the off gas is discharged through only the gas discharge hole 5a from the outer circumferential portion of the fuel electrode current collector 2, and it is possible to suppress the amount of the discharged fuel gas which does not contribute to the power generation reaction.
However, in the conventional technique, the fuel gas discharged from the outer circumferential portion of the fuel electrode current collector 2 and the oxygen-containing gas discharged from the outer circumferential portion of the oxidizing gas electrode current collector 3 easily contact in an area near the outer circumferential portion of the power generation cell 1. Therefore, the fuel gas and the oxygen-containing gas are combusted near the outer circumferential portion of the power generation cell 1, and local heat increase or the like occurs. Consequently, the power generation cell 1 may be damaged undesirably. Further, the insulating cover 5 is made of material which is heterogeneous to material of the power generation cell 1 or material of the oxidizing gas electrode current collector 3. Thus, because of the difference of the linear coefficient of thermal expansion, the contact resistance increases due to cracks or contact failure. As a result, the power generation efficiency and the durability may be degraded disadvantageously.